Though I have no experience with a JA200, my path down the road of other Jadis amplifiers has led me to the following opinions:
1) KT90 are a poor tube choice. The KT90 is a cold, sterile tube, which does not allow the magic of a Jadis to come through almost at all. Why did the company choose this tube? Well, they preferred the Tesla/Teslovak/JJ KT88 of the mid - late 90s, but the reliability of those tubes was such an issue that job number one is not to have the amplifier go down in the field
2) 6550 are not as good as KT88 tubes. Again, the KT88, in general, is a more romantic, emotional tube. You buy a Jadis to produce the romance and emotion of live music. It doesn't make much sense to use a tube which does not allow that
3) KT88 are a very good overall choice. The tube has a nice fat mid/upper bass that I like, with the sweetest presentation of the larger tubes.
4) EL34 are my favorite output tubes, with a midrange that is simply and startingly beautiful. The downside is that you are going to give up bottom end, to the point where it may be unacceptable. Eventually, through trial and error, I've found the key to resolving this is to find the right 12AX7 driver tubes. You can even produce thunderous bass by getting this right.
As an aside, the JJ K77 strikes an interesting balance between the EL34 and KT88. Its midrange is nowhere near as magical as the KT88, but it does trump most KT88 I have tried. And, the low frequencies seem to be as powerful as the KT88 in most regards, though they can be a bit bloated if other things aren't taken care of.
In conclusion, my choice for a Jadis is the EL34. However, if I didn't pick the right driver to account for the lack of bass, I'd go with a JJ KT77 (first) or one of the KT88 variants (secondly).
1) KT90 are a poor tube choice. The KT90 is a cold, sterile tube, which does not allow the magic of a Jadis to come through almost at all. Why did the company choose this tube? Well, they preferred the Tesla/Teslovak/JJ KT88 of the mid - late 90s, but the reliability of those tubes was such an issue that job number one is not to have the amplifier go down in the field
2) 6550 are not as good as KT88 tubes. Again, the KT88, in general, is a more romantic, emotional tube. You buy a Jadis to produce the romance and emotion of live music. It doesn't make much sense to use a tube which does not allow that
3) KT88 are a very good overall choice. The tube has a nice fat mid/upper bass that I like, with the sweetest presentation of the larger tubes.
4) EL34 are my favorite output tubes, with a midrange that is simply and startingly beautiful. The downside is that you are going to give up bottom end, to the point where it may be unacceptable. Eventually, through trial and error, I've found the key to resolving this is to find the right 12AX7 driver tubes. You can even produce thunderous bass by getting this right.
As an aside, the JJ K77 strikes an interesting balance between the EL34 and KT88. Its midrange is nowhere near as magical as the KT88, but it does trump most KT88 I have tried. And, the low frequencies seem to be as powerful as the KT88 in most regards, though they can be a bit bloated if other things aren't taken care of.
In conclusion, my choice for a Jadis is the EL34. However, if I didn't pick the right driver to account for the lack of bass, I'd go with a JJ KT77 (first) or one of the KT88 variants (secondly).